Electronic generator of telephone ringing current



ELECTRONIC GENERATOR OF TELEPHONE RINGING CURRENT Original Filed Aug.18. 1952 INVENTORS= GEORGE H. BRODIE ARTHUR J. RADCLIFFE JR.

ATTORNEY United States Patent O Claims. (Cl. 3151-75) This inventionrelates to an electronic generator of telephone ringing current. Themain object is to provide a simple, reliable, and efficient electronicgenerator suitable for supplying ringing current for use in a telephonesystem to operate the ringers at called telephones.

A further object is to provide a device which is readily adjustable tothe precise desired frequency and to provide a balanced wave form, andwhich is sufiiciently stable in frequency and waveform that it servessatisfactorily in frequency-selective party-line telephone systems.

In its preferred form, the invention includes a twintriode push-pullpower amplifier which is transformer- .coupled to the ringing currentoutput wires and which is 'driven by a suitable square-wave voltagegenerator, or vibrator. In the preferred form of the invention, thedriving voltage generator takes the form of a dual-triode electronicmultivibrator.

' A feature of the invention resides in the use of oppositely poledsources of power for the multivibrator voltage generator and the poweramplifier. This provision permits .the two parts to be directly coupledas in a direct-current amplifier, thereby lessening the loadrequirements on the multivibrator and consequently enhancing'itsstability of operation, as well as permitting the desired low-frequencyvoltage to be faithfully impressed on the grids of the power-amplifier.- I

This application is a continuation of our application Serial Number305,048, filed August 18, 1952, now abandoned.

A feature of the disclosed simple driving arrangement .',is that thegrid of either triode of the power amplifier is ;driven alternately to anegative cutoff potential and to cathode potential, rather than to onemore positive than the cathode, whereby a relatively high power-outputefliciency is realized without drawing grid current, thereby furtherlessening the power demand on the driving multifvibrator. Other objectsand features will become apparent as the ;.description progresses.

Referring to the accompanying drawing, the invention -;is illustrated asapplied to an electronic generator comprising a multivibrator voltagegenerator 1 and a push-pull amplifier 2, supplied with oppositepolarities of direct-current by power unit 3. 'zVoltage generator 1, byits controlled multivibrator action hereinafter described, provides theamplifier 2 with "voltage of the desired frequency and basic waveform,thus functioning as a driver for the amplifier 2, to which its output isdirectly coupled over wires 15, 16.

Power amplifier 2 has its output transformer-coupled .to a pair of busbars 4, from which the output ringing cur- ;rentlis supplied through therespective sections of a telephone switchboard (not shown) overcorresponding branch pairs of conductors such as 75 and 76 by Way ofrespective current-limiting lamps (generator lamps) such ,as 77 and 78.By way of example, telephone ringing current is commonly of a frequencyin the range extending from-- about 16 to 66 cycles per second. 1

The power unit 3 contains two oppositely poled sources is illustrated asbeing grounded. Terminal 5 is negative 'Ce 2,991,128 Patented July 4,1961 with respect to common terminal 6, whereas terminal .7 is positivewith respect to the grounded common terminal. Ground at the commonterminal 6 is extended to items 1 and 2 by common conductor 12;conductor 11 extends from terminal 5 to supply a negative 400-voltpotential to the voltage generator 1; and conductor 13 extends fromterminal 7, by way'of fuse 14, to supply a positive 25 O-volt potentialto power amplifier 3.

The voltage generator 1 comprises one form of the wellknown free-runningmultivibrator. It develops voltage of the desired frequency andmagnitude, and of a balanced square waveform to drive the poweramplifier 2 over wires 15 and 16. Generator 1 includes the twin triodetube 20, of which the two sections provide alternative paths for currentconduction from negative power-supply conductor 11, through loadresistors 24 and 28 to ground.

OPERATION OF VOLTAGE GENERATOR 1 As previously indicated, the voltagegenerator 1 comprises a free-running multivibrator. Such a multivibratorcomprises essentially two mutually coupled triode amplifiers. In theusual form, herein illustrated, the output of each triode amplifier iscapacity coupled (condensers 32, 34) to the input of the other,wherefore a signal causing conduction in one depresses conduction in theother by virtue of the out-of-phase voltage relationship between thegrid and anode of a triode amplifier.

The two triodes of voltage generator 1 comprise the two sections ofdual-triode tube 20. The left section includes cathode 25, control grid26, and anode 27, parts 21, 22, and 23 being the respectivelycorresponding parts of the right section.

The voltage generator 1 starts operating whenever the parts thereof areassembled, and interconnected, and supplied with power as illustrated,provided that the cathodes 25 and 26 are in heated condition, by virtueof current flow through their illustrated heaters from a suitable sourceof heater current (not shown). For simplicity of explanation, however,it may be assumed that power-supply wire 11 is disconnected from theheated cathodes 21 and 25, but is connected as illustrated to theright-hand terminal of potentiometer 29. Current is flowing from wire 11to ground through items 29 and 35, rendering the potential on the slidearm of item 29 somewhat less negative (more positive) than wire 11 to adegree depending on its setting. This relatively positive potential onthe slide arm of item '29 passes through the slide arm of potentiometer30, and through the end sections thereof and through grid resistors 33and 31, to grids 26 and 22, respectively. These grids are thus normallymore positive than negative supply wire 11.

If wire 11 now be connected as shown to the cathodes 25 and 21 of tube20, grid current immediately starts to flow from these cathodes to theirrespective grids. The high resistance of grid resistors 33 and 31 and ofthe respective halves of the balancing item 30 keep this flow small,resulting in the grids 26 and 22 being drawn down to a potentialsubstantially no more positive than the cathodes 25 and 21, thus placingthe grids at a definite working point at the positive end of thegrid-voltage control range. This is the point at which cathode-anodecurrent flows most readily.

With both grids at the positive end of the range, current immediatelystarts to build up in both sections of ,tube 20. This rise of currentbuilds up a negative potential (with respect to wire 12) on anodes 23and 27 by virtue of the drop across the associated resistors 24 and 28.This rising negative potential on the plate of either triode section 13is passed by condensers 34 and 32 to control grids 26 and 22. Theresulting negative potential on either control grid acts to inhibitconduction through the associated section of tube 20 to an extentdepending the two inhibiting actions fall short of exact equality, withthe result that the initial current rise is faster in one section thanin the other. The section through which the faster current rise occursdraws full current almost instantly, wherefore its anode reaches itsfull negative potential almost instantly, and therefore almost instantlystops conduction through the other triode section by driving the grid ofsuch other section negative beyond the cutofi point.

By way of example, if the left-hand triode section is the initiallyconducting one, the voltage drop thereby pro duced across load resistor28 causes the anode 27 to become highly negative with respect toconductor 12. The control grid 22, by the action of the couplingcondenser 32, simultaneously becomes substantially equally negative withrespect to its described normal potential supplied through grid resistor31. The resistors 28 and 24 are so chosen that the voltage drop acrosseither, when the associated section of tube 20 is fully conducting, issutficient to act through the concerned condenser 34 or 32 to swing thegrid of the other section of the tube negative beyond cutoff. Therefore,the right-hand section of tube 20 is rendered completely non-conductingby the sudden rise of current to full value through the left-handsection and by the described resulting negative swing of grid 22.

Full current continues to flow through the left-hand section of tube 20until the right-hand section starts to conduct, as it soon does byvirtue of the return of the potential of grid 22 toward normal potentialby the flow of leakage current through grid resistor 31.

At the end of a short time interval, the length of which depends, amongother things, upon the capacity of condenser 32 and the resistance ofgrid-leak resistor 31 and associated section of item 30, the excessnegative potential induced on grid 22 through condenser 32 hassufiiciently leaked off through resistor 31 that the right-hand section(cathode 21 and anode 23) start to conduct. The consequent initiallysmall negative drop across resistor 24 acts through condenser 34 torender grid 26 more negative than its described normal biased potential.Thereupon, the flow of current through the left-hand sectionresponsively decreases a corresponding amount. This initially smalldecrease in flow correspondingly decreases the negative drop acrossresistor 28, which acts through condenser 32 to render grid 22immediately less negative, thereby reinforcing the initially small flowof current through the right-hand section. The flow of current in theright-hand section is thus self-reinforcing by virtue of the describeddecreasing action on the flow in the lefthand section and the consequentpositive flow-assisting po. tential which is reflected back to grid 22by condenser 32. As a result, when the current initially starts to flowin the right-hand section of tube 20 by virtue of the leakingolf of thehigh negative potential from grid 22, the cur rent flow through suchsection rises abruptly to full value, accompanied by the abrupt decreaseof current to zero in the left-hand section. This sudden cessation ofcurrent in the left-hand section of tube 20 results from the suddennegative voltage drop across resistor 24, reaching grid 26 throughcondenser 34. Grid 26 is thereby driven negative beyond the cut-offpoint, as describedv hereinbefore for grid 22. The relatively positivepotential placed at this time on grid 22 through condenser 32 (bycessation of current flow through resistor 28) is limited in value tocathode potential by the grid current that flows when grid 22 tends tobecome positive. Either grid of tube 20 thus returns to its referencepoint at the positive end of the grid voltage range at the commencementof a period of current flow through its section of tube 20. Theleft-hand section again starts to conduct, when enough of the negativepotential (with respect to cathode) has leaked off grid 26. At thispoint the described reflecting action occurs to switch that section intofull conduction, at the 4 same time abruptly terminating flow in theopposed section.

The described flip-flop alternate conducting action of the two triodesections of tube 20 continues indefinitely, with either springingabruptly into full conductivity coincidentally with an abrupt cessationof flow in the other section.

When the device 1 is operating satisfactorily, the alternatelyconducting sections of tube 20 conduct for equal lengths of time, andconversely, remain non-conducting for equal lengths of time. Anyobserved inequalities in this respect may be remedied by adjusting theslide arm of balance potentiometer 30, which has the effect of add ingresistance to the leak path of one grid (26 or 22) and removing it fromthe leak path of the other.

When device 1 is operating satisfactorily, the frequency of operation(the number of times a second either tube conducts) is theringing-current frequency chosen for bus bars 4. Any observed departurefrom that frequency may be remedied by moving the slide arm ofpotentiometer 29 to the left to increase frequency and to the right todecrease frequency. For example, moving the arm of part 29 to the leftrenders it more positive, to thereby hasten the leaking off (throughresistor 33 or 31) the excess negative potential of the currentlyblocked grid.

For five selected frequencies in the ringing-current frequency rangefrom 16 to 66 cycles, the components of voltage generator 1, with theindicated 400-volt supply over wires 11 and 12, may be as follows:

Tube 20 may be a commercial twin triode of the type sold as 6SN7;resistor 35 may be 240,000 ohms; potentiometers 29 and 30 may be 50,000and 500,000 ohms, respectively; and resistors 24 and 28 may each be50,000 ohms. For the indicated frequencies, the condensers 32 and 34,and the resistors 31 and 33, may have values as listed in the followingtable:

Frequency table Cycles 32 and 34. 31 and 33, mt. megohml OPERATION OFPOWER AMPLIFIER 2 Power amplifier 2 responds to voltage and frequencycontrol received from voltage generator 1 over wires 15 and 16 toimpress alternating ringing current of substantial power and of thedesired ringing frequency on bus bars 4, by way of coupling transformer60. The cathodes 55 and 51 of the power-amplifier tube are connected toconductor 12, which is the positive anodesupply wire for themultivibrator tube 20. Through wires 1.2, 15, and 16, the load resistors24 and 28 of tube 20 are thus connected in the position of gridresistors for the respective sections of tube 50: resistor 24 connectsgrid 52 to its cathode 51; and resistor 28 connects grid 56 to itscathode 55. Therefore, when either resistor 24 or 28 is currentless, thecorresponding grid 52 or 56 is at the potential of its cathode 51 or 55,and the associated triode section of tube 50 conducts. But, when aresistor 24 or 28 is carrying current incidental to the describedmultivibrator action of device 1, the associated grid of tube 50 isthereby rendered highly negative with respect to its cathode, whereuponcurrent flow through the concerned triode section of tube 50 is entirelyblocked.

Since, as described, the two sections of tube 20 conduct alternately,the grids 52 and 56 are rendered negative alternately, and the twonormally conducting sections of power-amplifier tube 50 are therebyrendered non-conducting alternately.

When the -left-hand section is non-conducting, as it is the potential ofthe cathodes 55 and 51 reaches grid 52 "through wire 12, resistor 24,and wire 15. Theconduc- -tion path through tube 50 is now from theintermediate terminal6 of power supply 3, over conductor 12, cathode 51,anode 53, terminal 62, the lower half of the primary winding oftransformer 60, terminal 63, fuse 14, and over conductor 13 to thepositive 250-volt terminal of power source 3. The core structure oftransformer 60 is thereby magnetized in a given direction.

A moment later, the right-hand section of tube 50 becomes nonconducting, and its left-hand section becomes conducting, by virtue ofcurrent flow ceasing in load resistor 28 and starting in resistor 24.Current flow thus ceases in the transformer section lying betweenterminals .62 and 63, and starts in the section lying between terminals64 and 63, over the following path: from the intermediate terminal 6,conductor 12, cathode 55, anode 57, terminal 64, associated winding,terminal 63, and thence through fuse 14 and conductor 13 to the positive250- volt terminal 7. The core structure of transformer 60 is therebymagnetized in the opposite direction.

The secondary winding of the transformer 60 responds to the notedmagnetization of its core in opposite direc tions by impressing analternating-current potential across bus bars 4, thus serving as asource for supplying telephone ringing current over the branch circuitssuch as 75 and 76. This ringing current is of balanced wave form and ofthe frequency generated by device 1. The condenser 71 shunting thesecondary winding renders the voltage waveform across bus bars 4 morenearly sinusoidal.

With no load applied to the power amplifier 2 by the bus bars 4, the twohalves of the primary winding of transformer 60 appear highly inductiveto the anode current in each half of the primary, and as a result ofthis inductance, limit the anode current to a low and safe value. As theload applied to the bus bars 4 becomes greater, the effective inductanceof the primary becomes less, whereby the current-limiting action isdecreased, allowing a. greater anode current to flow. When the appliedload on bus bars 4 becomes great enough to cause damage to the poweramplifier 2, fuse 14 opens, rendering the power amplifier 2 inoperativeand causing an alarm system (not shown) to operate.

For any of the selected frequencies in the ringing-current frequencyrange from 16 to 66 cycles, the components of power amplifier 2, withthe indicated 250-volt supply, over conductors 12 and 13, may be asfollows: tube 50 may be a commercial twin-triode such as the 6AS7; fuse14 may be Mi ampere; condenser 71 may be .040 Inf; and transformer 60may have a primary consisting of two balanced sections of 1710 turnseach and may have a secondary of 1260 turns.

The voltage generator 1 and the power amplifier 2 hereinbefore describedmay be one of a plurality of such combinations as required in atelephone system employing selective frequency ringing. The power pack 3may serve as a source of power for a plurality of the described devicesas required. As indicated in the drawing, conductors 11, 12, and 13 maybe in multiple, to supply the respective potentials to as manyhereinbefore described devices as desired.

Referring again to the multivibrator voltage generator 1, it will beobserved that the described operation thereof results in the negativewire 11 of the power unit 3 being connected alternately to the two wires15 and 16 which lead to the grids of power-amplifier 50, and that theresistors 24 and 28 keep these grids normally supplied (over wire 12)with the potential of the cathodes of tube 50, with the result thateither grid of the power tube is i at zero potential with respect to itscathode except when supplied directly with a high negative potentialthrough the associated section of the tube 20, at which time theassociated resistor 24 or 28 draws current.

that the tube 20 could be replaced by a suitabletuned It is evidentmechanical switching vibrator adapted to connect wire 11 to wires 15 and16 alternately, but the multivibrator 'tube arrangement is presented asbeing more economical and more stable in operation, A mechanicalswitching vibrator, however, lacks the undirectional limitation of thetube 20, wherefore the wires 12 and 11 incoming to device 1 could bereversed if tube 20 were replaced by a mechanical vibrator. Then thegrids of tube 50 would pull power amplifier including two sections eachhaving .a control grid and having a cathode and an anode for carryingpower current subject to the associated control grid being at apotential with respect to its cathode which is less negative than acertain cutoff potential, a con trol source of direct current of apotential in excess of said cutofi potential having its positive poleconnected to said cathodes, means for applying potentials from the polesof the current source to respective ones of said grids in a given and ina reversed pole to grid cor.- respondence alternately at a desiredalternating-current frequency, whereby the grids are operated in phaseopposition between cathode potential and a potential more negative thansaid cutoff potential to cause the two sections of the amplifier toconduct alternately, means for causing the applying means to perform itssaid operations continuously at a substantially constant predeterminedfrequency, and means for causing the applying means to effect abruptlyeach of said change of pole to grid correspondence, to thereby applysubstantially a square-wave form of voltage change to each grid, eachamplifier section being thereby caused to undergo a substantiallysquare-wave form of change between substantially zero conductivity andmaximum conductivity.

2. An alternating current generator according to claim 1, furthercomprising grid resistors continuously connecting the said gridsrespectively to one pole of the said current source, the said applyingmeans comprising means for connecting the other pole of the said currentsource to the grids alternately at the said substantially constantpredetermined frequency.

3. An alternating-current generator according to claim 2, in which thepole to which the said resistors continuously connect the said grids isthe said positive pole of said current source, and in which the saidother pole which is connected to the grids alternately is the negativepole of the said current source.

4. An alternating-current generator according to claim 3, in which saidmeans for connecting the said negative pole to the said gridsalternately comprises a pair of control triode sections connectedrespectively between the said grids and said negative pole, togetherwith means for controlling the respective control elements of thecontrol triode sections to cause the last said sections to conductalternately only, the last said means including means for impressing asubstantially square-wave form of controlling change separately uponeach of the last said control elements to effect substantiallyinstantaneously each change of condition of the last said sections fromthat of conducting to that of non-conducting, and conversely.

5. An alternating-current generator comprising a pushpull poweramplifier including two sections each having I including two triodesections each-comprising a control grid, a cathode, and an anode, twolinking conductors directly connecting the anodes of the multivibratortriodes to the grids of the respective power-amplifier sections, twoload-impedance devices for the multivibrator sections each having oneterminal connected to a separate anode thereof, a return conductorconnecting the other terminal of each load-impedance device in common tothe cathodes of the power-amplifier sections, a power-supply conductorfor the multivibrator sections extending in common to the cathodesthereof, a source of direct-current potential in excess of said cutoffpotential having its positive pole connected to said return conductorand having its negative pole connected to said power-supply conductor,bias-conducting means for normally maintaining the control grid of eachmultivibrator section at a potential with respect to its cathode whichwill render conducting the space between the associated cathode andanode, two condensers each connecting a separate anode of themultivibrator sections to the control grid of the other multivibratorsection, said condensers being of such capacity that the multivibratorsections are rendered non-conducting alternately at a selectedfrequency, whereby the resulting potential drop occurring alternatelyacross said impedance devices causes the potential of thepower-amplifier control grids with respect to their associated cathodesto vary in phase opposition between a potential which is more negative,and a potential which is less negative, than said cutoff potential.

References Cited in the file of this patent UNITED STATES PATENTS2,154,492 Clough Apr. 18, 1939 2,297,926 Usselman Oct. 6, 1942 2,529,459Pourciau et a1. Nov. 7, 1950 2,552,968 Hochwald May 15, 1951 2,579,374Edwards Dec. 18, 1951 2,629,006 Oliver Feb. 17, 1953 2,639,379 BlancherMay 19, 1953

